The present invention relates to multi-band communication systems and in particular to multi-band Ultra Wide-Band systems.
Recently, intense attention has focused on Ultra Wide-Band (UWB) systems, which can offer very high data rate of more than 110 Mbps over a short range of about up to 10 m for broadband wireless applications including wireless multimedia stream or wireless video connection. UWB systems are systems, which use extremely large bandwidths. In the past, such systems were only used in military applications. However in 2002, the Federal Communications Commission (FCC) in the US allowed the use of the 3.1-10.6 GHz band for commercial ultra-wideband applications. Furthermore, the FCC defined that an ultra-wideband signal must occupy at least 500 MHz bandwidth or have a fractional bandwidth greater than 0.25. To generate such large bandwidths of up to 7.5 GHz, various methods exist including short pulse, chirp modulation, frequency hopping and the like.
Typical pulse generated ultra-wideband systems transmit a short pulse followed by a gap with no transmission until the next pulse is sent. The rate at which the pulses including the subsequent time gap between pulses are sent is known as the pulse repetition frequency (PRF). If the pulses of such a UWB system occupy one very broad band (from 500 MHz up to 7.5 GHz), these systems are called Single-Band UWB Systems. If the pulses occupy several smaller bands of more than 500 MHz, these systems are called Multi-Band UWB systems.
The block diagram of FIG. 1 shows an example for a multi-band UWB transmitter. An impulse generator provides impulses to a pulse shaping filter, which is e.g. implemented using a low-pass or band-pass filters. The output of a pulse-shaping filter is a pulse shaped impulse signal. The mixer up-converts the pulse shaped impulse signal to the desired operating centre frequency. The bandwidth of the UWB signal at the output of the mixer is determined by the bandwidth of the pulse-shaping filter. The centre frequency as well as the instantaneous phase of the UWB signal can be controlled via oscillator control. A RF band-pass filter is used at the output of the mixer to reject undesirable or out-of-band frequencies and/or mixer products prior to a transmission via an antenna. A more detailed description of an UWB transmitter is e.g. given in U.S. Pat. No. 6,026,125.
The adjustable centre frequency of the oscillator depicted in FIG. 1 enables multi-band UWB system with frequency hopping. Frequency hopping patterns for multi-band UWB system have been proposed by Discrete Time in “Discrete Time PHY Proposal for TG3a, IEEEE802.15-03/099r1, March 2003”, Intel in “IntelCFP Presentation for a UWB PHY, IEEE802.15-03/109r1, Mar. 3rd 2003”, Philips in “Philips TG3a CFP Presentation, IEEE802.15-03/125r2, Mar. 2nd 2003”, and General Atomics in “General Atomics Call for Proposal Presentation, IEEE802.15-03/105r1, March 3rd 2003” as part of the contributions for a UWB PHY to be developed in IEEE 802.15-3a. Whereas Intel and Discrete Time propose using hopping patterns to avoid persistent collision of co-located un-coordinated piconets (network of devices connected in an ad hoc fashion using Bluetooth technology) by using the pattern to differentiate the piconets, Philips and General Atomics propose using hopping patterns as information bearing signal, i.e. the pattern itself is used to encode data.
An example for the impulse response of the pulse-shaping filter in FIG. 1 is a Gaussian window. Mathematically the Gaussian window w(t) is defined as:
                              w          ⁡                      (            t            )                          =                  ⅇ                      -                                          t                2                                            2                ⁢                                  σ                  2                                                                                        (        1        )            with t=0 being the centre of the pulse window and σ the standard deviation.
To prepare a baseband signal for a transmission over a defined frequency band, the baseband signal is usually multiplied with a sine wave of the centre frequency of the frequency band. In multi-band UWB system the sine wave is multiplied with a Gaussian window to result a pulse on the respective frequency band. This mixing is mathematically described by:x(t)=s(t)·w(t)whereby s(t)=sin (2πft) and
      w    ⁡          (      t      )        =            ⅇ              -                              t            2                                2            ⁢                          σ              2                                            .  x(t) is the signal at the output of the mixer, s(t) is the sine wave and w(t) is the same Gaussian window as in equation (1). In case of fixed bandwidth the standard deviation σ of the Gaussian window is the same for all frequencies f. In FIG. 2 three pulses with different centre frequencies flow, fmedium, and fhigh are shown. These pulses can be observed between the mixer and the band-pass filter of FIG. 1. Since all of the three pulses have the same duration, all of them are occupying the same bandwidth at different frequencies. Those pulses with equal length are used in a system with sub-bands of a fixed bandwidth.
Since the number of cycles per pulse is different for each pulse, the auto-correlation properties of the three pulses are different. FIG. 3 shows the spectrum of a multi-band UWB system with seven sub-bands of a fixed bandwidth. As can bee seen, the roll off and the bandwidth of all sub-bands are the same.
FIG. 4 shows a typical staggering of frequency sub-bands defined for a multi-band UWB system. Only one sub-band is used at a time and by a pulse with the respective centre frequency. In the example shown, a frame consists of seven pulses with each pulse transmitted in the next higher sub-band to that of the previous one. Strictly speaking, the first pulse of a frame is transmitted over the sub-band ‘1’ with the centre frequency f1, the second over the sub-band ‘2’ with the centre frequency f2, and so on until finally the seventh and last pulse of the frame is transmitted over the sub-band ‘7’ with the centre frequency f7. The order by which the pulses are sent over the respective sub-bands is referred to as pulse transmission order. In the example shown it is defined as (1, 2, 3, 4, 5, 6, 7). In a more general definition, the pulse transmission order corresponds to a definition of a sequential order in which the frequency bands defining the signal transmission channel are to be used for up-converting a baseband signal.
This pulse transmission order has a detrimental effect on the adjacent channel interference. The adjacent channel interference results from spectral overlapping of the pulse frequencies, as depicted in FIG. 3, and the multipath characteristic of a typical radio channel. As can be seen, the spectra of pulses with adjacent centre frequencies overlap and this overlapping results in adjacent channel interference if the pulses are received at the same time. This will occur even if each pulse is sent at a different time because the pulses are delay-spread by the multipath of a mobile radio channel. The effect of delay-spread is illustrated in FIG. 5. The transmitted pulse reaches the receiver via several paths with different delays and attenuations due to reflections and shadowing effects. The resulting signal at the receiver consists of multiple copies of the originally transmitted pulse but each copy with a different time delay, phase and amplitude. A respective received signal is shown on the right hand side of FIG. 5.
The adjacent channel interference resulting from a corresponding multipath propagation of the pulses is illustrated in FIG. 6. The centre frequencies of the pulses shown in FIG. 6 are assumed to be those indicated in FIG. 3. The grey shaded areas show the time periods, where pulses with adjacent centre frequencies interfere with each other at the receiver. The shown is of particular relevance for multipath channels which have a delay spread of the same order than the pulse repetition frequency (PRF). What has been described with reference to a multi-band UWB system applies quite in general also to any frequency hopping multi-band system.